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Date: Thu, 28 Jan 2016 18:58:15 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Daniel Walker <danielwa@cisco.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>, Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, "Khalid Mughal (khalidm)" <khalidm@cisco.com>,
        "xe-kernel@external.cisco.com" <xe-kernel@external.cisco.com>
Subject: Re: computing drop-able caches
Message-ID: <20160128235815.GA5953@cmpxchg.org>
References: <56AAA77D.7090000@cisco.com>
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On Thu, Jan 28, 2016 at 03:42:53PM -0800, Daniel Walker wrote:
> "Currently there is no way to figure out the droppable pagecache size
> from the meminfo output. The MemFree size can shrink during normal
> system operation, when some of the memory pages get cached and is
> reflected in "Cached" field. Similarly for file operations some of
> the buffer memory gets cached and it is reflected in "Buffers" field.
> The kernel automatically reclaims all this cached & buffered memory,
> when it is needed elsewhere on the system. The only way to manually
> reclaim this memory is by writing 1 to /proc/sys/vm/drop_caches. "

[...]

> The point of the whole exercise is to get a better idea of free memory for
> our employer. Does it make sense to do this for computing free memory?

/proc/meminfo::MemAvailable was added for this purpose. See the doc
text in Documentation/filesystem/proc.txt.

It's an approximation, however, because this question is not easy to
answer. Pages might be in various states and uses that can make them
unreclaimable.